1. Field of the Invention
The present invention relates to a PM stepping motor, and more particularly to a PM stepping motor with limited size.
2. Description of the Related Art
FIG. 5 shows an example of a typical PM stepping motor (hereinafter referred to as “stepping motor” as appropriate). A stepping motor of FIG. 5 generally includes two stators 6A and 6B, and a rotor 9.
Each of the two stators 6A and 6B includes an outer yoke 2 having a cylindrical outer frame 2f and a plurality of pole teeth 2a, an inner yoke 3 having a plurality of pole teeth 3a intermeshing with the plurality of pole teeth 2a of the inner yoke 2 on the same circumferential plane, and a bobbin 4 having a coil 5 therearound and housed in an annular hollow space formed by the outer yoke 2 and the inner yoke 3. The two stators 6A and 6B thus structured are coupled to each other back to back.
The rotor 9 includes a shaft 8 and two magnets 7A and 7B disposed on the shaft 8 axially separate from each other, and is rotatably disposed inside the stators 6A and 6B such that the outer circumferences of the two magnets 7A and 7B oppose the pole teeth 2a and 3a of the stators 6A and 6B with a small air gap therebetween.
In recent years, a photographing or filming equipment, such as a digital or video camera is becoming widely popular, and the performance of an automatic focusing mechanism for use with lenses in such the equipment has been remarkably enhanced while the mechanism dimension is significantly reduced. Under such the circumstances, there is an increasing demand for a small stepping motor with an outer diameter of 10 mm or less as driving means for such the automatic focusing mechanism (refer to, for example, Japanese Patent Application Laid-Open No. 2006-254557).
The above described stepping motor with a limited diameter is disadvantageous in terms of torque production but nevertheless is required to produce a necessarily ample output torque at a high rotation speed range. For example, a typical stepping motor for use with an automatic focusing mechanism for a digital camera, which has an outer diameter of 10 mm or less, generates a pull-in torque T of 1.2 g-cm or more at a frequency (rotation speed) f of 1000 pps, and it is desired for such the small stepping motor to produce an equivalent or larger torque even at a higher frequency f, e.g. 2500 pps, in order to assure a high speed response.